1. Field of the Invention
This invention relates generally to an electrical oscillator circuit and, more particularly, to a noise optimized ring oscillator design and method for using the random distribution of multiple oscillator signals to supply a low-noise oscillator signal product.
2. Description of the Related Art
Conventionally, oscillator signals are used in transmitter and receiver circuits in the processes of sending or receiving signals which communicate information. Noise that becomes mixed in with the communicated signal necessarily degrades the performance of the communication link, so that information becomes misinterpreted, the link margin between communicating parties decreases, or both. Oscillator signals are used with mixers in the up-conversion and down-conversion of carrier signals, or as reference clocks in phase-locked loop circuits to modulate or demodulate baseband signals. To maximize the performance of the communication link the oscillator signals are generated as free of noise as reasonable.
There are several types of noise that affect an oscillator signal. Coherent noise sources, such as neighboring frequency sources or power supplies create periodic signals which can be added to, or modulate the oscillator signal. Generally, these noise sources are addressed with shielding, line filtering, and adequate grounding. Other forms of noise are non-coherent or random. As is well known in the art, many components generate noise in response to temperature in accordance with the kBT formula, where T is temperature (Kelvin), k is Boltzmann""s constant, and B is bandwidth (Hz). This thermal, or Johnson, noise is gaussian in response and independent of frequency. Shot noise refers to the noise generated by active devices and is expressed as qIB, where q is an electron charge, I is dc current, and B is bandwidth. It is known that this noise response is also gaussian. Low frequency noise, also called contact or flicker noise, is also gaussian and proportional to the inverse of the frequency of the noise.
In oscillator design, noise analysis is often expressed in terms of phase noise. Generally, phase noise describes the shape of the sidebands of the intended oscillator signal. The steeper the sidebands surrounding an oscillator signal, the lower the phase noise. Sideband noise is typically introduced into an oscillator feedback loop, where its frequency modulates the oscillator signal.
Noise can be minimized with the use of a high Q resonant element. Q is defined as the ratio of the center frequency to the bandwidth. Alternately, Q can be expressed as the ratio of the impedance of the frequency dependent resonant elements, capacitors and inductors, to resistance. A crystal is a highly stable resonant element with a Q in the thousands that is used in many oscillator designs. The Q of a typical inductor/capacitor resonant circuit is orders of magnitude less. As is known in oscillator design, the random and coherent noise is divided by the Q of the basic resonant elements to yield the overall phase noise response of the oscillator. Thus, a crystal oscillator circuit will generate a signal having much less phase noise that an oscillator using standard inductors and capacitors.
FIG. 1 is a conventional ring oscillator circuit 10 (prior art). The ring oscillator 10 is an inverting circuit made by series connecting logic gates. The output of the gates is 180 degrees out of phase from the input signal at low frequencies. Delay through the series connected gates becomes a factor at higher frequencies. When the overall delay equals 180 degrees, the output signal is in phase with the input signal, and as long as the circuit 10 has an overall gain, the circuit will oscillate when the output is feed back to the input of the oscillator. These ring oscillators are easy to build. They can be fabricated merely by connecting a few gates or transistors in series. Since the circuit must invert, an odd number of inverting gates can be used. As shown in FIG. 1, three inverting buffers 12, 14, and 16 are shown.
Tuning elements can be added between stages to control the frequency of oscillation. Often the parasitics of the gates are used as frequency tuning elements. Since the tuning and parasitic elements typically have relatively low Qs, the overall Q of the oscillator is low. Further, the reactances of these components are not necessarily stable, so that the frequency of resonance is subject to change. These tuning elements and component reactances account for the inherently poor Q of a ring oscillator circuit.
FIG. 2 is a precision ring oscillator (prior art). A precision ring oscillator 20 is fabricated using delay elements, where the overall circuit 20 acts as an inverter at low frequencies. As shown, the precision ring oscillator 20 is formed with three delay buffer sections 22, 24, and 26. Delay buffer 26 is a variable delay buffer including high speed and low speed delay sections 28 and 30, respectively. The output frequency is sampled at phase detector 32 and compared to a reference frequency generated by reference clock 34. The phase detector controls the operation of the high and low speed delay sections 28 and 30 to precisely control the output frequency.
It would be advantageous is a circuit could be designed to supply a low-noise oscillator signal from a higher noise oscillator source signal.
It would be advantageous if the noise factor of a ring oscillator circuit could be improved.
It would be advantageous if ring oscillator performance could be simply enhanced in an integrated circuit simply using more of the same ring oscillator components.
Likewise, it would be advantageous if the ring oscillator performance enhancement could be accomplished in the same region of the IC as where the ring oscillator is fabricated, without the necessity of running traces to other sections of the IC, or off chip.
Accordingly, an integrated circuit low noise ring oscillator is provided. The ring oscillator comprises a plurality of ring oscillator sections, where each ring oscillator section has an input and output, both connected to the inputs and outputs of the other ring oscillator sections. That is, the outputs of the ring oscillator section are summed together and supplied as a common input, so that the ring oscillator sections are connected in parallel.
Each ring oscillator includes a plurality of n delay sections. Each delay section has an input to accept a signal and an output to provide a signal that is delayed at the frequency of operation (oscillation). The delay sections are series connected, with the output of one delay section connected to the input of a subsequent delay section. The first series connected delay section is connected to the input of the ring oscillator section to accept the input signal, and the nth delay section is connected to the output of the ring oscillator to provide the output signal.
A method is also provided for reducing noise in an integrated circuit ring oscillator signal. The method comprises: generating a first oscillator signal at a first frequency; generating a second oscillator signal at the first frequency; summing the first and second oscillator signals; and, supplying a low noise output oscillator signal at the first frequency.
Generating the first oscillator signal at a first frequency includes generating a signal having a first relative noise factor. Likewise, generating the second oscillator signal at a first frequency includes generating a signal having a first relative noise factor. However, supplying the output oscillator signal at the first frequency includes supplying the output oscillator signal at a second relative noise factor, less than the first relative noise factor. The output oscillator signal relative noise factor that is calculated as follows:
relative noise=first relative noise factor/(the total number of summed oscillator signals)xc2xd.